The [original] serial number/plate are to authenicate the year the tractor was made, thus validate the actual age of the tractor.
The more original parts a tractor has, thus the serial# plate, the more valuable the tractor is alleged to be. Of course anything today can be reduplicated for the right $$$ value. However, most of the tractor people i have dealt with are pretty honest and would state if the s-plate were changed/altered.
If you are just needing a tractor for your own personal use/consumption and showing it is not an objective, then a s-plate doesn't mean much...if however you are showing it, then your value as a tractor is considered less than the same type of tractor that has it original s-plate.
Happy New Year,